Let those of you who have hitherto been robbers now become soldiers. Those of you who have formerly contended against their brothers and relatives now fight against the barbarians, as they ought.
-Pope Urban II
The above quote was meant to draw the barbarian peoples of Europe together to help defend the Middle Eastern interests of Christendom (the Holy Land) from the predations of the Fatimid caliph Al-Hakim. Great Christian shrines and temples were destroyed and pilgrims slaughtered or tormented, and the Byzantine Empire put out the call for help.
And boy did they regret it.
This is not to say that I approve of Holy Wars (nor am I Catholic), but I am fascinated by the power that such rallying cries have over the hearts of mankind. Theres something extra-ordinary about rallying the masses to get up and fight for something worth fighting for... At least in theory.
Unfortunately, as seen in the case of the Crusades, the forces called to a righteous response to injustice are rarely worthy of the call, or even well-meaning by it. Many who responded to the call for crusades were themselves monsters (a good enough reason to export them I suppose). A Holy War usually translates to an excuse for rapine and plunder.
Still, such speeches are the spice of fantastic storytelling. Stirring speeches before a suicidal charge, rallying chants to get an otherwise bereft force to stand and make a last stand that ends up winning the day. War that is glorious and righteous rather than profane and wretched.
Sometimes it even happens in real life.
"This," he said, "is how I see the matter; if fight we must, let us make preparation to sell our lives dearly.
-Xenophon, The March Upcountry